Adenylate Kinase fromSulfolobus acidocaldarius:Expression inEscherichia coliand Characterization by Fourier Transform Infrared Spectroscopy

Adenylate kinase from the extremely thermoacidophilic archaeonSulfolobus acidocaldariushas been overexpressed inEscherichia coli.The highly purified enzyme was characterized by Fourier transform infrared spectroscopy (FTIR). Analysis of FTIR spectra and estimation of secondary structure revealed a global protein structure similar to that of other adenylate kinases. Thermal unfolding of the protein with an estimatedTmvalue near 90°C is irreversible due to protein aggregation. The enzyme exhibits long-term stability up to 80°C, which is an excellent adaptation to the physiological growth temperature of 75–80°C. Half-widths of secondary-structure-sensitive bands and hydrogen–deuterium exchange experiments revealed that in comparison to adenylate kinase from porcine muscle cytosol theSulfolobusenzyme is characterized by a significantly more compact and rigid protein core structure, which is likely to contribute specifically to the extreme thermostability of the protein.